1. Field of the Invention
This invention relates to polyurethane resins and is more particularly concerned with polyurethane resins both cellular and non-cellular having improved structural strength properties and with methods for their preparation.
2. Description of the Prior Art
The preparation of polyurethanes, both cellular and non-cellular, by the reaction of organic polyisocyanates, polymeric polyols, and low molecular weight extenders such as glycols, alkanolamines and diamines, is so well-known in the art as not to require detailed description herein.
Polyurethanes hitherto available have been used extensively in the fabrication of a wide variety of components, particularly the thermoplastic polyurethane elastomers which can be fabricated by injection molding or by reaction injection molding (RIM) techniques. However, the use of these materials to prepare components having structural strength properties which match those derived from engineering thermoplastics such as nylon and the like, has been limited by the need to provide extensive reinforcement using materials such as fiberglass in order to achieve desirable levels of stiffness, impact resistance and related properties.
We have now found that polyurethanes with markedly improved structural strength properties can be prepared by departing significantly from the previous teachings of the art as to the relative proportions of reactants to be employed. Thus, we have found that very substantial reduction in the amount by weight of the polymeric active-hydrogen containing material (e.g. polyol) employed in the preparation of the polyurethanes is a major factor in producing a highly surprising and dramatic change in the properties of the resulting polyurethanes. The change in properties is enhanced by selection of particular combinations of reactants as will be discussed in detail below. These changes enable us to produce resins which can be employed, without the necessity to incorporate reinforcing fillers and the like, to produce structural components which possess all the desirable impact resistance, stiffness, and other structural strength properties which have been achievable heretofore by the use of other polymers such as nylon and other engineering thermoplastics but not by polyurethanes alone.
To the best of our belief it has not been recognized that such a result as that described herein could be achieved. Thus it has been well-known since the inception of the polyurethane art that linear polyurethanes could be obtained by reaction of organic diisocyanates with one or more low molecular weight diols and or diamines; see, for example Otto Bayer, Angewandte Chemie, A59, No. 9, at pp. 255-288, September 1947; see also U.S. Pat. Nos. 2,284,637, 2,284,896, 2,511,544 and 2,873,266. Such products have found utility chiefly in the fiber field. It was also recognized early in the history of polyurethane chemistry (see Otto Bayer, supra) that a wide variety of useful products both cellular and non-cellular, could be obtained by reaction of organic polyisocyanates, polymeric polyols and low molecular, di- or polyfunctional active-hydrogen containing compounds such as glycols and the like (usually termed extenders). These products are obtained by the one-shot procedure by reacting all the components together simultaneously or by the prepolymer method which involves prereacting the organic polyisocyanate with a portion or all of the polymeric polyol and then reacting the resulting isocyanate-terminated prepolymer with the low molecular weight extender and any polymeric polyol which was not used in preparing the prepolymer. In general, the above types of product are prepared using a combination of polymeric polyol and low molecular weight extender in which there is at least one equivalent, and more usually several equivalents, of the low molecular weight extender for each equivalent of polymeric polyol. However, since the molecular weight of the polymeric polyol is substantially higher than that of the extender, the proportion by weight of the polymeric polyol used in preparing the polyurethane is substantially in excess of the proportion by weight of the low molecular weight extender.
The relative proportions of the polymeric polyol to the low molecular weight extender used in preparing such polyurethanes greatly influences the properties of the polyurethane which is obtained. Thus the polymer chain units derived from the low molecular weight extender are referred to as "hard segments" since they are relatively rigid, i.e. they exhibit high moduli of elasticity. The polymer chain units derived from the polymeric polyols are referred to as "soft segments" since, because of the presence of the relatively large polyol residues, particularly where the polyol is a linear polymeric diol, they exhibit low moduli of elasticity. In the case, for example, of a relatively linear polyurethane, prepared from a diisocyanate, a polymeric glycol and difunctional extender, increasing the proportion of extender to polymeric polyol gives progressively more rigid polyurethanes and, beyond a certain point, the polymer becomes relatively brittle and shows very low impact resistance when fabricated in the form of structural components.
In further illustration of this point it has generally been conventional, in order to prepare thermoplastic polyurethanes with reasonable levels of impact resistance, to avoid the formation of brittle polymers by maintaining sufficiently high proportions of soft segments (i.e. by using a substantial proportion by weight of polymeric polyol) and to generate a desirable level of stiffness by incorporating reinforcing fillers such as glass fibers into the polymer. However, this leads to additional problems caused by the difficulties of handling such mixtures particularly when the molding operation is being carried out as part of a RIM process.
Accordingly, we believe that it is all the more surprising to find that it is possible to choose certain combinations of organic polyisocyanate, polymeric polyol (or like active-hydrogen containing material) and low molecular extender in which the level of the polymeric polyol (i.e. the level of soft segments in the resulting polyurethane) is reduced to a very low order of magnitude and the level of the extender (i.e. the level of hard segments in the resulting polyurethane) is increased beyond a point at which it would have been expected that the resulting polymer would be too brittle, and to obtain products which are suitable for the fabrication of component parts having satisfactory structural strength including impact resistance.
While the substantially reduced level of polymeric polyol or like active-hydrogen containing material employed in preparing the compositions of the invention is one of the major characteristics of the latter which distinguish them from products hitherto known in the art, it is to be understood that there are additional considerations, to be discussed in detail below, which serve to differentiate these compositions very clearly from the compositions hitherto described in the art which is known to us.